1. Field of the Invention
The present invention relates generally to a demodulator for a frequency encoded data signal, and more particularly, to a means and process for determining the transition of the data signal between a first and second predetermined signal frequencies.
2. Description of the Prior Art
Frequency modulation is often applied to the transmission of binary data signals. Depending upon the binary notation, two frequencies are utilized so that during the modulation, impulses are formed with a period determined by the respective signal frequency. Data signals transmitted in such a form are typically fed to a limiting amplifier contained in the signal receiver, which amplifier generates a square wave signal.
Previously, the amplified and frequency-modulated signals have been demodulated by two oscillating circuits one of which resonates at one of the selected signal frequencies while the other resonates at the other selected frequency. Then, a low-pass filter arrangement connected on the output side of the oscillators transmits the demodulated data.
Other previously known devices demodulate the two signal frequencies by means of a filter which is turned to a frequency chosen between the two signal frequencies.
The primary disadvantage of the previously known processes employing oscillating circuits or filter circuits is that the data signals in these circuits are oftentimes distorted by transient phenomena, i.e. by building up and decaying processes. Any satisfactory evaluation within very short time intervals is impossible since the respective actual signal information is distorted by the preceding information which still remains as a signal voltage due to the storage effect of capacitors and inductors. Thus considerable difficulties are encountered when striving for a high digital data transmission rate such that the time interval for one bit reaches the order of magnitude of a period of the respective signal frequency. Moreover, in the case of the filter process, a complicated multistage filter is required when the evaluation of the transitions between the two signal frequencies must be in the proper phase relation.
The determination of the exact phase of the transition between the two frequencies is especially important for phase-coherent signals. In this case, during a high speed transmission rate, upon a frequency transition there still remains a transmission signal on the carrier frequency line capable of distorting the data signals to an especially high degree.
Still other previously known devices utilize a zero-passage discriminator for the demodulation of frequency-modulated signals. Such a discriminator counts the signals received per unit time as they pass through the zero point. The number of zero passages per unit time thus defines the frequency of the respective signal. However, the zero-passage discriminator is unsuitable when the two signal frequencies differ only by a small amount and/or when high ratios of transmission rates to signal frequencies exist.
Other previously known devices measure the time interval between consecutive passages of frequency-modulated signals through the zero point. This elapsed time is compared with a predetermined threshold value which represents the normal duration of the signals received for one frequency. These devices also suffer from the disadvantage that in case of a small difference between the modulating frequencies, the accuracy of the evaluation is diminished. In addition, these devices are unduly expensive when the proper in-phase relation of the transition between the frequencies must be determined.
For the demodulation of a frequency-modulated signals, other previously known devices also measure the half period of the signals by using a counter with a constant clock cycle which during the respective period, is repeatedly counted up to its maximum count. Depending upon the count obtained at the end of the respective half period, a demodulated signal is generated via a low-pass filter arrangement, which signal denotes the respective assigned frequency. Also, with these devices, it is difficult to obtain a proper in-phase evaluation, especially of phase-coherent signals.
These previously known processes all have the common disadvantage that the reproduction of the data signals always requires a low-pass filter arrangement triggered by signals generated by oscillating circuits or digital counters. In particular, these devices are disadvantageous in that signal distortions are caused by transient phenomena from inductors and capacitors.